508 POPULAR SCIENCE MONTHLY. 



the earliest times, the fact that they undergo periodic changes in 

 number and size was not recognized until long after the discovery of 

 the telescope, and the law of the change was not determined with any- 

 thing like accuracy until the middle of the last century. Accurate 

 observations of their number and size were only commenced about 

 1830, but from about 1750 to 1850 enough material had been gathered 

 to show that a maximum occurred on the average every eleven years. 

 In the last two or three decades the use of the spectroscope has added 

 greatly to our knowledge of their nature and motions, and the photo- 

 graphic camera has enabled astronomers to add permanent records of 

 the state of the sun's surface at any time to the numerical estimates 

 which were the chief contribution of the earlier observers. Such 

 pictures are of special value whenever, as in the case of sunspots, much 

 depends on the personal equation of the observer and still more on the 

 particular method used in forming an estimate of them. Thus there 

 is a period extending over the last seventy years in which continuous 

 observations have been made, a period of another seventy years earlier 

 in which the observations are worth discussing, although much less re- 

 liance can be placed on them for accurate deductions, and a still earlier 

 period of about a hundred years from which a little doubtful informa- 

 tion can be gleaned. 



The net result which has been deduced from this series is an average 

 period of 11-g- years between the maxima or times of greatest sunspot 

 activity. But this average period by no means represents all the facts. 

 The time between two consecutive maxima has been as long as 17 

 years and as short as 8 years, while some maxima are marked by much 

 larger and more numerous spots than others. The average period be- 

 tween the minima or dates of fewest sunspots is rather less irregular, 

 since it has never been known to differ by much more than two years 

 from the mean period of 11 years. There is some evidence that 

 these stronger and weaker maxima themselves run in a cycle, but the 

 period is very doubtful. Wolf, who devoted most of his life to the 

 investigation of sunspots, deduced a period of 55 years for this longer 

 cycle, while there are later determinations of 6o, 35^ and 61 years. 

 Further, the curves representing the number of spots from day to day 

 or from month to month show many other irregularities which have 

 up to now defied any attempt to group them in any regular order. 

 Thus we have a well-marked average period of about 11 years and 

 doubtful ones of about 35 and 60 years. These are therefore the 

 periods into which the meteorological records are to be grouped in order 

 to inquire whether there is any connection between the two sets of 

 phenomena. 



There is good reason to believe that a period of about 35 years is to 

 be traced in certain of the meteorological records, and therefore it is 



